1. Field of the Invention
This invention relates to hollow cathode plasma switches and switching methods.
2. Description of the Related Art
Solid-state switching devices have previously been developed which include gate-turn-off thyristors and integrated-gate-bipolar-transistors. These devices are capable of rapid switching, low voltage drop and cryogenic operation, and have been used in invertor/converter systems that convert high power from a low to a high DC voltage. However, the solid-state switches must operate at fairly low voltages (less than 1 kV), and their transformer coupling to high voltage outputs is poor at high step-up ratios in excess of 10. They are also subject to catastrophic failure under single over-currents or over-voltages, and cannot operate in high temperature/high radiation environments.
A low pressure plasma opening switch that overcomes these disadvantages of solid-state switches is referred to as the CROSSATRON modulator switch (CROSSATRON is a trademark mark of the Hughes Aircraft Company, the assignee of the present invention). Details of this switch are provided in U.S. Pat. No. 4,596,945, issued June 24, 1986 to R. W. Schumacher et.al., and assigned to Hughes Aircraft Company.
The CROSSATRON switch is a secondary-electron-emitter, cold cathode device that employs a controlled diffuse discharge to both close and open pulsed power circuits at high speed and high repetition frequency, enabling operation at substantially higher voltages and currents than solid-state switches. In addition, the CROSSATRON switch is rugged, fault tolerant and can be cooled cryogenically. However, it typically produces a relatively high forward voltage drop on the order of 500 volts, which makes it unsuitable for low-source voltage applications of less than about 5 kV.
A later plasma switch was developed that retains the advantages of the CROSSATRON switch, but operates with a much lower forward voltage drop (on the order of 20 volts), and higher system efficiency. This device, referred to as a Hollotron switch (Hollotron is a trademark of Hughes Aircraft Company), is described in co-pending U.S. Pat. application Ser. No. 07/406,673, filed Sept. 13, 1989 by Robert W. Schumacher et.al., and assigned to Hughes Aircraft Company. It uses a thermionic hollow cathode discharge to form a dense xenon plasma which provides a low forward voltage drop during conduction. The switch also includes a grid-controlled current interruption feature to provide fast, square-pulse modulation.
A drawback of the aforementioned HOLLOTRON switch is that it relies on geometric expansion of the hollow cathode plasma to provide a sufficiently reduced density for interruption. This approach limits switching to approximately 2 amps of peak current at a current density of about 2 amps/cm.sup.2. As the current is increased above this level, the higher plasma density generated in the switch is accompanied by a pinching or area constriction ("filamentation") of the plasma's current-carrying channel, which in turn prevents interrupting the current to open the switch. The inability to interrupt current at the higher current levels is believed to be due to Debye shielding of the interruption voltage in the control grid apertures. Simply constructing larger hollow cathodes or moving the control grid and anode further away from the hollow cathode does not assure larger plasma areas or lower plasma densities, and does not increase the switch's current capacity. The plasma channel which carries the current from the dense plasma formed in the interior of the hollow cathode tends to self-pinch to a small cross-section because the plasma channel exhibits a negative resistance, and because there is a finite inward JxB force from the neutralized electron current flowing in a plasma. The high plasma density limits the current and current density that the switch can handle.
Hollow cathodes were originally developed to replace hot filaments in electron-bombardment ion sources to obtain longer life, higher current, and lower power consumption. A typical hollow cathode developed for use in ion thrusters is described in W. Kerslake, D. C. Byers, and J. F. Staggs, AIAA Paper No. 67-700. 1967. This type of hollow cathode was used in the HOLLOTRON switch described above. Operation of a hollow cathode as a plasma source in the magnetic-field-free region of an ion source is described in D. M. Goebel et.al., "Plasma Studies on a Hollow Cathode, Magnet Multipole Ion Source for Neutral Beam Injection", Rev. Sci.Instrum., Vol. 53, No. 6, June 1982, pp. 810-815. In this case, as in ion thruster geometries, the hollow cathode is used as an electron source to generate a discharge for production of ions and ultimately the formation of an ion beam. The hollow cathode is positioned opposite a negatively biased ion accelerator, and the region between is enclosed by a chamber wall biased at anode potential.
Magnetic fields are typically employed in ion thrusters to improve the ionization efficiency of the discharge. In this application, a secondary ionization region (discharge chamber) is positioned between the hollow cathode plasma source and the beam extraction grid. This region is generally bounded axially by two flat plates which are biased at cathode potential, and bounded radially by an electrode (the anode) which is biased at a positive potential with respect to the hollow cathode. A magnetic field is employed primarily to prevent electrons from proceeding directly to the anode from the hollow cathode plasma without first experiencing energetic collisions with neutral gas atoms and thereby generating additional ionization.
In the ion source described in the Goebel et.al. article, there is no mechanism provided to disperse the highdensity plasma stream from the hollow cathode aperture. The filamented plasma channel from the hollow cathode extended over 20 cm into the ion source. To disperse the pinched plasma stream by collisions and produce a uniform plasma at the ion extraction electrode, the ion source had to be constructed with a length from cathode to ion accelerator of over 40 cm. This long length resulted in significant plasma loss to the anode walls, a relatively high voltage drop of typically six times the ionization potential, and a modest overall efficiency of the device.
The problem of dispersing the plasma stream from a hollow cathode was addressed in the early stages of ion thruster development. In the work described in H. J. King et.al., "Electron-Bombardment Thrusters Using Liquid-Mercury Cathodes", J. Spacecraft and Rockets. Vol. 4, No. 5, May 1967, pp. 599-602, a diverging magnetic field was used in part to spread the plasma from a mercury hollow cathode over a large area at the beam extraction grid. Nevertheless, a non-uniform, strongly peaked-on-axis density profile was produced.
Because electrons emitted from the hollow cathode are electrostatically confined between the cathode and first accelerator grid, and magnetically confined such that radial loss to the anode is impeded, electrons are forced to diffuse radially to the cylindrical anode via collisions and E.times.B instabilities. Although the increased ionization rate improves discharge efficiency, the long diffusion distance for the ionizing electrons to travel from the axis of the source to the anode tends to result in the non-uniform, strongly peaked-on-axis plasma profile. To eliminate the highly peaked-on-axis plasma profile in ion thrusters, a baffle was placed on axis directly in front of the hollow cathode. The axial magnetic field was retained to provide the electron confinement from the anode and increase the ionization efficiency. The baffle forces the electron discharge to run off-axis to provide increased plasma density at the outer radius of the beam extraction grid, while electron-plasma collisions allow the discharge chamber plasma to fill in the hollow profile downstream of the baffle.
There are several geometries of such ion thrusters in the literature. These are described in an article by H. R. Kaufman, "Technology of Electron-Bombardment Ion Thrusters", included in Advances in Electronics and Electron Phvsics, ed. L.Marton, Vol., 36, Academic Press, 1974, pp. 266-373. The shaped magnetic field and baffle combination produce uniform plasma densities at the ion accelerator grid, but raise the discharge voltage from anode to cathode to more than twice the ionization potential. In fact, the baffle geometry is normally optimized to raise the discharge impedance to increase the ionization efficiency.
The general ion source configuration with a hollow cathode and a diverging magnetic field was also investigated at Oak Ridge National Laboratory, and is described in C. C. Tsai et.al., "Plasma Studies on a DuoPIGatron Ion Source", Rev.Sci.Instrum., Vol. 48, No. 6, June 1977, pp. 651-655. To produce uniform plasma over larger areas (10 cm to 30 cm diameter), it was also necessary to insert an on-axis baffle at the hollow cathode aperture and add additional magnetic confinement by surface multipole magnetic fields at the anode wall.
The purpose of the magnetic field in all of these devices is primarily to enhance the ion production rate (discharge efficiency) in the discharge chamber outside the hollow cathode and secondarily to produce a uniform ion current to the acceleration electrode. The magnetic field shape in the baffle region is usually optimized to purposely raise the discharge voltage to several times the ionization potential to increase the ionization efficiency of the discharge.